Can someone explain the correct placement of 'almost' in the sentence
The almost is a modifier. In "almost 20%" it modifies 20% meaning that the number jumped to a figure close to 20%. Written the other way, "jump almost by 20%" it modifies almost which is not the intended meaning.
"Jumped at" is not idiomatic and "jumped to" is correct so this cuts out A, D and E. Leaving B and C.
In C "who" has no referent. It is supposed to refer to citizens but that is not the subject, the subject is "the number".
So B is correct.